How to prepare a sheet for welding?

Once upon a time, I helped a colleague in patching the car sills. According to him, the reparations should be cut only with sheet metal scissors, because if I did it with a grinder, I would overheat the cut, changing the properties of the metal, which would make it worse to weld, and then be susceptible to rust. Tinsmiths welders - is it true? And if so, then the bare rust on the threshold was cut with a grinder anyway - should I also cut it with scissors?

Once upon a time, I helped a colleague in patching the car sills. According to him, the reparations should be cut only with sheet metal scissors, because if I did it with a grinder, I would overheat the cut, changing the properties of the metal, which would make it worse to weld, and then be susceptible to rust. Tinsmiths welders - is it true? And if so, then the bare rust on the threshold was cut with a grinder anyway - should I also cut it with scissors?

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